Śatapatha Brāhmaṇa ("of a hundred paths") 2.5.2.6

2.5.2.6

sa uttarasyāmeva vedau | uttaravedimupakirati na dakṣiṇasyāṃ kṣatra vai varuṇo viśo marutaḥ kṣatramevaitadviśa uttare karoti tasmāduparyāsīnaṃ kṣatriyamadhastādimāḥ prajā upāsate tasmāduttarasyāmeva vedā uttaravedimupakirati na dakṣiṇasyām

On the northern (uttara) altar he raises the uttara-vedi (upper or north altar), not on the southern one. Varuna, doubtless, is the nobility, and the Maruts are the people: he thus makes the nobility superior (uttara) to the people; and hence people here serve the Kshatriya, placed above them. This is why he raises the uttara-vedi on the northern, not on the southern altar.